Control system for gas burners



1366- 7 P. F. SWENSON CONTROL SYSTEM FOR GAS BURNERS Filed June 27, 1952INVENTOR. P401. F SWEA/SOA/ A T7'0/?/V v nited States Patent CONTROLSYSTEM FOR GAS BURNERS Paul F. Swenson, Cleveland Heights, Ohio,assignor, by mesne assignments, of thirty-seven percent to Otto Wanek,two percent to Kenneth J. Kitchen, ten percent to Paul F. Swenson, tenpercent to Marcus 0. Swenson, four percent to Myron T. Cooperrider,eleven percent to Wilma G. Stupka, eleven percent to Wilma G. Stupka,trustee, ten percent to J. A. Weeks, and five percent to Earl P.Schneider, trustee Application June 27, 1952, Serial No. 295,850

Claims. (Cl. 236-80) The invention relates to certain improvements inauto matic controls and specifically to an efiicient and safenon-electrical control system and apparatus for gas burners.

The objects, aside from those already indicated, include the provisionof an automatic burner control system which operates efl'iciently andsafely to obtain, subject to simple adjustment, either completemodulated-heat operation or on-off operation of a burner or acombination of both; a gas burner control system wherein subatrnosphericpressure maintained by flow of the gas through a pilot burner apparatusis utilized effectually and safely fully to control the rate of flow ofgas for main burner operation and for shutdown in event of derangementor malfunctioning of the burner apparatus or failure of the gas supply.

The burner mechanism disclosed herein as adapted for control by thepresent system is on the order of those shown in Swenson et a1, Patent2,572,675 issued October 23, 1951, and Swenson et a1. application Ser.No. 37,347 filed July 7, 1948, now Patent No. 2,664,153.

Specific objects will be indicated in connection with the followingdescription of the illustrative forms shown in the drawing, wherein:

Fig. 1 is an assemb y, largely schematic and partly sectional viewshowing the principal parts of the burner and of the elements invol edin its control.

Figs. 2 and 3 are detail fragmentary, partly sectional, views of ametering valve and pilot thermostat protective feature taken asindicated at 2--2 and 3-3 on Fig. 1.

Fig. 4 is a sectional view showing a type of non-metallic tubing whichthe present control system is uniquely adapted to utilize as asignal-transmission line.

The gas burner assembly A of Fig. l is arranged for use in an updraftboiler or other combustion unit. A typical installation for such usecomprises an upright main burner head B at the base of a combustionchamber structure C, the burner head forming the outlet portion of anL-shaped gas and primary air mixer tube T suitably mounted within aburner enclosure box or air supply duct E connected to the bottomcombustion chamber wall around the burner head. All the air needed forcombustion is admitted to the duct E through a single air inlet openingcontrolled by an automatically adjusted damper or door D. Variablyadjustable gas metering valve M through a single nozzle orifice 0, seeFig. 2, introduces main burner gas into the flared mouth of the mixertube, approximately centrally of the inlet end of the tube, as aconcentrated substantially constant velocity single stream. Appropriateamounts of primary air from within the box E are thus drawn into thetube T,

are there mixed with the gas and the mixture is con-- trollably admittedto the combustion chamber througha variable flame port F formed betweena movable lid or cap L and the adjacent rim of the upright enlarged endportion of the mixing tube. Secondary air from the box or duct E,admitted by damper D, enters the combustion chamber through a sleeveportion Q of the box around the burner head.

Gas admitted through a manually operated cock PG from an assumedregulated source supplies a pilot-light assembly P adjacent to theburner head. Another manual cock G, beyond pilot gas cock PG in thedirection of gas flow, admits gas from said source to the inlet side ofa burner input or main flow control or throttle valve V, shown as aflexible diaphragm. Valve V is the output element of a vacuum-signalledrelay valve mechanism R and, in connection with the illustrated burnermechanism, the valve V acts as a master in that, in accordance with itsposition, a gas-pressure-actuated slave motor S operates, through a rodS and connected linkage, automatically to adjust or set gas meteringvalve M, variable flame port F and the air inlet control damper D aswill be more fully explained later.

The control system, as shown, includes an aspirator device 10 of thepilot flame assembly P for establishing a partial vacuum in a signaltransmitting vacuum line, represented by a tube 11, as a function of gasflow to the pilot burner; various valve devices such as 12, 13 and 14which are operable to admit air under normal pressure into the tube 11(vacuum varying or breaker valves) and a vacuum-responsive orsignal-receiving spring biased diaphragm mechanism 15 (upper rightFig. 1) which is the input (pilot valve adjusting means) of relay valveunit R in automatically controlling the position of valve V.

The aspirator 10 is incorporated in a fitting 16, a sleeve 17 of whichsupports a pilot burner tube 18 having a ported pilot flame cap ornozzle piece 19 for maintaining a pilot flame adjacent to the mainburner flame port F. One eflective end of the tube 11 is sealed aroundan aspirator-chamber-iorming bore 19' of the sleeve 17 and, as shown,the tube is communicated with the chamber through several small radialholes 20 intersecting the chamber. Pilot light supply gas issuingthrough a small orifice 21 at the base of the sleeve 17 draws air fromthe tube 11 and maintains subatmospheric pressure in the tubeproportional to the rate of flow of gas to the pilot burner. Such ratemay be assumed to be approximately constant and of the desired value atfully open position of the cock PG. Primary air for the pilot light isadmitted through openings 22 in the sleeve 17.

Through the operation of mechanism not yet described the size of theflame at the main burner is maintained proportional to the degree ofvacuum effective on the top side of a highly flexible or delicatelyacting diaphragm 23 forming a movable wall of a vacuum chamber 24 ofsignal-receiving mechanism 15 of relay R to which chamber a portion ofthe tube 11 is suitably connected. Diaphragm 23 is biased by a low ratemodulating spring 25 in a direction to oppose the vacuum force whichtends to lift the diaphragm.

Main flow control or throttle valve V of relay valve mechanism R, asshown, includes a suitable sealing disc 27 on a diaphragm 28 shown inFig. 1 in closed position on its seat, i. e. the position which blocksmovement of gas from the source to the outlet chamber of the valve Vfrom which the gas issues to slave motor S and the metering valve M. Agenerally closed chamber 30 above the diaphragm 28 of valve V isestablished, in part, by a partition 31 and its supporting casing walls.A bleeder 32 conducts gas from the inlet side of valve V into thechamber 30 normally to maintain the valve V closed by pressuredifferential on the diaphragm. The under side of the vacuum-responsivediaphragm 23 carries a sealing plug or pad 36 in valving relation to theupper end of a venting tube 34 for the chamber 30 and which extends intoa continually vented or open chamber or space 37. The passage throughthe tube 34 is continually open into chamber 30 and the lower endportion of the tube is designed to rest on a central rigid portion ofthe diaphragm 28 of valve V. An elastic double flanged seal 35 aroundthe bent tube 34 blocks flow of gas from chamber 30 along the outer wallof the said tube into venting chamber 37 and also forms a pivotalsupport for the tube. Chamber 37, in order not to Waste gas ventedthrough the bent tube 34 may be connected to the main burner mixer tubeas by a pipe 38.

With the above described arrangement of relay mechanism R, it will beapparent that, as the vacuum-responsive diaphragm 23 is lifted bypressure differential at its opposite sides a sufficient distance topermit venting of chamber 30 through the control or pilot valve port atthe top of the bent tube 34 at a rate exceeding the gas conductivecapacity of the bleeder 32, the gas pressure below diaphragm 28 opensvalve V, and that the bent tube 34 transmits the valve opening motion asfeedback to the pilot valve in accordance with well establishedprinciples of relay or servomechanism operation- As one example of adetector device which is settable to vary the pressure difference insideand outside of the signal transmitting tube 11, or, for convenience, therelayoperating vacuum in the tube, the instrument 13 is shown as aroom-type thermostat having a freely pivoted bimetal lever 1.3a carryinga valving element 13b in restricting or sealing relation to an orificeor control port of the tube (e. g. tube end as shown) until the bimetalelement is subjected to a predetermined space temperature rise, andwhich then opens or varies the eflective area of the port or orifice inproportion to the amount of increase in space temperature. Thus, at agiven setting of an adjusting cam 13c, the control port is variablyrestricted in accordance with space temperature decrease, and therelay-operating vacuum in the entire tube is proportionally increased.

When the vacuum-actuated mechanism 15 is adjusted for full rangemodulating operation the modulating spring 25, acting downwardly on itsassociated vacuumresponsive diaphragm 23, as through a flanged sheetmetal cup 40, exerts a relatively small downward force sufficient toclose the pilot valve port formed by the top end of the venting tube 34,so that, when gas is admitted to the system through cock G, the valve Vwill be closed by gas pressure action as already described. Now assumingthe pilot light gas is turned on at valve PG, producing a partial vacuumin the tube 11 through aspirator 10, and that the bimetal lever 13a ofthe detector mechanism 13 has closed its vacuum line control port, orhas restricted it a certain amount, it is evident that the diaphragm 23of mechanism 15 will be lifted by atmospheric pressure in the chamber 37until the increasing modulating spring force balances said pressureforce. Thus the opening movement of the valve V is steadily limited bythe feedback lever action of venting tube 34, to restrict the pilotvalve port. Restricted venting through the tube 34 during modulatingoperation of the burner continues to take place substantially at therate gas flows through the bleeder orifice 32.

Gas supplied to the burner mechanism A at a controlled rate throughthrottling relay valve V causes steady operation of the burner in amanner to satisfy the detected requirement for heat through operation ofslave motor S and its connected mechanisms already identified and whoseoperation will be more fully explained later.

If, in vacuum-responsive mechanism 15, the modulating spring 25, throughthe intermediary of its supporting cup 40, is permitted to follow allmovement of the diaphragm 23, as assumed in the above description, andthe spring 25 is properly designed as to rate and scale true modulatingburner operation takes place, which is to say that, within the capacityof the burner, only such an amount of gas is admitted through themetering valve as will continue to balance the heat loss in the spacebeing heated. In cases such that, (e. g.) through limitations in fuelquality or pressure or design limitations of the installation served,modulating operating at very low flame setting is undesirable, thecontrol system can be converted to on-01f operation at or below adesired minimum flame setting by temporarily holding the modulatingspring out of action so that the vacuum-responsive diaphragm 23 has noeffectual stabilizing force acting on it (no variable rate). Settablescrew device 50 extending from the casing of mechanism 15 through thecup 40, the screw device 50 having a head portion 51 underhanging thetop Wall of the cup, enables setting of the mechanism for such on-offoperation as will be apparent. When set for on-oil?" operation theweight of the diaphragm or its weight combined with that of a helperdisc 52 assures maintenance of the plug element 36 in sealing contactwith the pilot valve port of the venting tube 34 when the burnermechanism is required to be shut down. As depletion of pressure invacuum chamber 24 of mechanism 15 occurs, the diaphragm 23 snaps intocontact with the lower rim of the cup 40, gas is admitted to the burner,and modulated operation then takes place only at higher than the minimumflame setting which would otherwise be obtainable.

Thus the apparatus affords variable on-off operation in combination withmodulated operation, or, if desired, modulated operation only.

Instrument 12, is illustrative of one type of limit control which may beused to break the vacuum in control line 11 and terminate burneroperation in the case of an emergency. A bellows 12a of a vaporgenerator, further represented by bulb 12b, operates to push a ball typeplug out of sealing relation to a port in a wall of the vacuum line 11thus causing shutdown of the burner. The ball is self returning tosealing position. The same (vacuum breaking) operation is performed bythermostat device 14 associated with the mixer tube T. Here a bimetalelement 14a inside the tube displaces valve ball 14]) out of itsport-sealing position in event of flashback of main burner flame intothe mixer tube. Shutdown in that case continues only until the bimetalelement 14a cools off, and the duration of the cycle is determined bydesign.

Referring to the burner pilot light apparatus (see Figs. 1 and 3) asafety device 55 associated with the aspirator 10 etc. preventscommencement of burner operation in event the pilot flame isextinguished when burner heat is demanded as by room temperature changedetector instrument 13. A bimetal element 56 of the pilot lightapparatus has as shown, a release finger positioned to move valve ball57 off its seat on a vacuum-line-connected port when the bimetal elementcools. An important feature of the device is represented by a pilotflame deflector and shield 59 for the bimetal element 55. The shield iscarried and positioned by the bimetal element (when cool) inflame-directing position, (Fig. 3), in respect to one or more smalllateral flame ports 18:; of the pilot burner tube which ports establisha flame ladder along the tube between the sleeve 17 and cap 19. Intimateassociation between the pilot flame and the bimetal is highly desirableat commencement of burner operation, (pilot just lighted) because thecontrols cannot function until the valve ball 57 is seated. However,thebimetal material deteriorates,

rapidly by oxidation if left in contact with flame. As

soon as the bimetal element becomes heated and the shield is therebymoved a short distance to the left from its flame-deflecting position(Fig. 3) into any of various positions it may occupy during main burneroperation, the flame is cut ofl from access to the bimetal. A barrier 59is recommended as indicated in Fig. 1 to shield the bimetal element 56from main burner heat.

While any suitable pipe can be used to form the various portions of thecontrol tube 11, installations requiring long go and return portions canbe facilitated by the use of (e. g.) plastic twin tubing stock such asillustrated (cross section only) in Fig. 4. Such go and return tubingportions 11a of any appropriate shape, preferably circular, are madereadily separable from each other as at a connecting frangible webportion 11b formed as a part of the twin tubing. One advantage, asidefrom cost, of using plastic tubing is that if, during installation, itis accidentally bent shut at any point, as in leading of it aroundcorners, it is more apt to spring back than metal into properlyfunctioning condition.

In the following description of the burner mechanism A, which involvesfrequent mention of relatively low and high B. t. u. content gas (orsimply B. t. u. gas), reference is to differences in the essentialchemical composition of the gases discussed.

For use of the burner A, as controlled by the above described system,interchangeably with relatively low and high B. t. u. content gas and ininstallations which differ from each other radically in design it isnecessary, in order to avoid interchangement of parts and for reasonsexplained below, to provide for adjusting the operating ratio betweenthe slave motor S and the metering valve and, separately therefrom, foradjusting the automatic operating effect of the slave motor on thecommon connections with the variable flame port and air control damper,and, additionally, for adjusting the damper independently of all otheradjustments.

Low B. t. u. gas, throughout the control range, needs a relatively openmetering valve aperture or area of orifice O as compared to the apertureareas which are suitable for high B. t. u. gas for a given heat output;the flame port must be more restricted for high B. t. u. gas because ofthe more rapid flame propagation rate of such gas particularly where ahigh turn down ratio is desired (stability at low flame settings as forautomatic modulated control from a relatively high flame setting), andsecondary air must be properly and accurately limited if the burner isto operate with fairly high efficiency i. e. be free from elfective heatoutput reduction (stack loss) by excessive secondary air. Proper mixtureof gas with primary air is of course highly important, regardless of thetype or grade of gas which is used, and the problem of obtaining optimumgas/ air ratio and mixture velocity at the flame port for all flamesettings becomes more critical with higher B. t. u. gas i. e. the higherheat content.

The metering valve M and its operating slave motor S as herein arranged,constitute an inherently stable mechanism as will be seen from thefollowing: Gas is supplied through suitable passages at throttled rates(function of variable position of valve V) from an assumed regulated orsubstantially constant pressure source to the diaphragm chamber 44 ofthe slave motor S and simultaneously to the metering valve chamber whichis formed between a body 60 and metering valve plug 61. The diaphragm 44of the slave motor, (shown as connected to the metering valve plug 61 bya lever 62 having a movable supporting fulcrum 63 and adjustingmechanism 65 for it), opens and variably restricts the metering valve.orifice O in accordance with the position of the diaphragm 44 in itschamber. Gas-pressure-responsive movement of the diaphragm in themeteringvalve-opening direction is opposed by an adjustable biasingmeans, shown in the form of a diaphragm-attached tension spring 66having supporting and adjusting screw elements 67 and 68 one of which isconnected to the spring. Since the diaphragm chamber 44' and themetering valve M have a common source of supply of gas, the action ofthe diaphragm 44 becomes stabilized, in a position determined by theadjusted force of spring 66, as increasing volume issuance of gas fromthe metering valve nozzle orifice 0 enables reduction of the net forceacting on the diaphragm 44 to zero.

Largely to enable the burner to be operated with a stable flame at lowgas setting, (high turndown ratio), and interchangeably with relativelyhigh and low B. t. u. content gas, the metering valve M, as more fullyexplained in Swenson et a1. application Ser. No. 37,347, is so arrangedas to preserve a nearly constant velocity and direction of the singlestream of gas issuing from it into the mixing tube, assuming a generallyconstant supply pressure.

The velocity at low gas throttle settings is required in order to causea local mixing and accelerating of a quantity of air of only that volumewhich is needed for combustion of the gas and the directing of suchlocal gas-air-mixture within the mixing tube to the flame port F.

When the metering valve has only two parts, body 60 and plug element 61,an extremely effectual shape for the metering orifice, whereby the crosssectional form of the gas stream is uniform regardless of its size andthe boundary surfaces defining the orifice can be minimized ingas-fiow-restricting effect is a triangle, (may be diamond shape, i. e.two triangles). The illustrated metering orifice O is of triangularshape and, because of it, the orifice dimensions vary progressively intwo directions with opening and closing movement of the valve. Thusincrements of metering orifice area increase as a function of equalincrements of slave motor movement (non-linear functions of slave motormovement).

In order to maintain eificient combustion ratios of gas and air at allgas throttle settings it is necessary that the flame port and air supplyareas shall at all times be properly varied in relation to the volume ofgas being supplied through metering valve M. A simple manner of makingthe opening and closing movements of flame port cap L and air damper -Dnon-linear functions of diaphragm movement to about the same degree asthe metering valve gas volume adjustment is non-linear comprises, asshown, conversion of the rectilinear motion of the diaphragm connectedrod S into rotary motion of a crank 70 as by a rack and gear couple 71,72 and to use such a portion of the crank throw for operation of aflame-port-adjusting link 73 and air-damper-adjusting link 74 thatdownward movement of the diaphragm 44 out of the illustrated initialposition (metering valve, flame port and air damper closed) results in aprogressively increasing rate of opening movement of the linkoper-atedparts substantially matching the rate of increased metering valveopening area with opening movement of the metering valve. In a properlydesigned arrangement (showing being largely schematic) the attachment ofboth links 73 and 74 to the crank at or close to the effective deadcenter initial position of the crank relative to the links and the cranksupporting axis secures approximately optimum automatic coordination ofair inlet, flame port and metering valve control.

For adjusting the amount of air which is variably admitted by the damperD independently of flame port and metering valve adjustment and control,and particularly to meet the requirement for less secondary air in somecases than in others, due to special draft effects of installationdesign or to the B. t. u. content of the gas which has to be used, aswill be explained, the point of attachment of link 74 (pivot bracket 75)is settable toward and away from the damper hinge 76 by appropriatemeans. The motion of the link 73, as shown, is communicated to theflame-port-adjusting cap L through a counterweighted bellcrank 78 andpushrod 79 to which 75' latter the cap is secured.

Adaptation of the burner for use interchangeably with gases havingwidely different B. t. u. content involves changing the operating ratiobetween the slave motor S and the metering valve M so that (e. g. ingoing from low to high B. t. u. gas) a smaller metering valve orificefor a given heat output occurs when using such high B. t. u. gas ascompared with the orifice size which is required for low B. t. u. gasand the same heat output, and, additionally, for efiicient combustionover a wide control range, reducing the flame port and air duct inletareas as compared to those which are suitable for low B. -t. 11. gas.

Since the pressures, at which dilferent common types of gas (differingfrom each other in chemical composition) are used, normally increasewith increasing B. t. u. content, the slave motor/metering valveoperating ratio mentioned above can be adjusted according to a fixedscale (not shown) associated with the lever fulcrum adjustment means 63,65. Thus, from a position of the fulcrum 63 appropriate to a typical lowB. t. u. gas, the fulcrum is shifted toward the metering valve (e. g. asa first operation) for use with a known higher B. t. u. content gas. Theautomatic flame port (and air damper) control is then coordinated withthe adjusted automatic metering valve movement by increasing thediaphragm-biasing force of the spring 66, usually by trial manipulationof the nuts 68 with the gas throttled to a desired low flame setting,until, due to the higher pressure of the higher B. t. u. gas acting onthe diaphragm, the flame port and air inlet are sufliciently restrictedto secure or generally insure a steady low flame. The higher pressure,hence kinetic energy, of the higher B. t. u. content gas assists, at lowflame, in maintaining the necessary velocity of the single gas streamfrom the metering valve orifice O for localized entrainment of air inthe mixing tube and conveyance of the mixture to the flame port.

It was indicated above that both installation design peculiarities andgas composition have pronounced effect on draft hence burner efliciency.This refers (e. g.) to stack pull variations in updraft types ofequipment such as shown and to differences in draft-modifying flameintensely, varying with the B. t. u. content of the gas. Notwithstandinglimitation of secondary air supply as by damper adjustment at 75 to aminimum for peak efficiency of the burner, the single source of air ascontrolled by the damper does not tend to cause the gasair mixture inthe mixing tube to be ineffectual providing that the gas stream as itissues from the metering valve orifice has adequate air-inspiratingvelocity at all times (assured by the disclosed metering valve and itscontrols as already explained) and that substantially uniform airpressure is always maintained at the two ends of the mixing tube. Suchuniform air pressure results from the fact that the restriction at theair damper determines the air pressure in the entire air duct Eincluding that within the secondary air outlet sleeve Q and, of course,the inlet end of the mixing tube T which lies within the duct E inadequately spaced relation to its walls. An uncontrolled primary airsource outside of the duct E to the inlet throat of the mixing tubewould, under high draft conditions, frequently cause the mixture issuingfrom the flame port to be too lean for stable combustion.

I claim:

1. In a gas burner control system and apparatus, a burner and a gassupply conduit leading thereto having gas inlet and outlet passages, aport between the passages and a movable valve member capable of closingand opening the port, means forming a generally closed gas receptionchamber defined in part by said movable valve member, the receptionchamber being in restricted communication with the gas inlet passage forinlet-gas-pressure-operated closing of said port, wall means forming avacuum chamber and means remotely thereof and capable of inducingsubatmospheric pressure-therein, signal means 8 remotely of the vacuumchamber and operative to vary subatmospheric pressure therein, one wallportion of the vacuum chamber being movable in response to the varyingsubatmospheric pressure in the vacuum chamber, and gas conduit meanscommunicating with the gas reception chamber at one end and having itsopposite end disposed to be sealed and unsealed by said movable wall ofthe vacuum chamber, said opposite end, when unsealed, being arranged tovent gas from said reception chamber to atmosphere at rates in excess ofthe restricted rate of reception of gas thereinto from the said gasinlet passage whereby to enable opening of said port.

2. Apparatus according to claim 1, wherein a burner gas flow ratemodulating spring is operatingly connected to the movable Wall of thevacuum chamber so as to oppose with increasing spring force movement ofsaid movable wall of the vacuum chamber in a direction to unseal saidgas conduit means for communication with atmosphere.

3. Apparatus according to claim 2, having, additionally, adjustableabutment means associated with the spring in a manner to render thespring ineffective to oppose initial movement of said movable wall inits conduit-meansunsealing direction.

4. In a control system for a main gas burner having a pilot light burnerwith a gas supply independent of the control system, means forming acontrol passage, means operable to-maintain a partial vacuum in saidcontrol passage, gas control valve means for the main burner, and vacuumactuated means connected with the control passage and acting to initiateopening of the control valve means only when a predetermined degree ofvacuum exists in the control passage, a main burner safety devicecomprising a normally closed and self closing vacuum-breaker valve inthe passage, a bimetal element associated with the pilot burner, meansfor supporting the bimetal element so as to expose a freely movableportion of the element to pilot burner flame heat, said portion beingoperatingly disposed in relation to the vacuum-breaker valve so as toopen that valve only when the bimetal element is unheated by pilotburner flame whereby the main burner will not be supplied with gas whenthe pilot burner flame is extinguished, and a deflector device carriedby said freely movable portion of the bimetal element, said device, inan unheated condition of that element, being disposed in a position todirect heat of the flame toward the element, and, when the element hasbeen heated to the point of permitting the vacuum breaker valve toclose, is then interposed between the pilot flame and said element in amanner to shield the element from flame heat.

5. In a gas burner control system and apparatus, a burner and a gassupply conduit leading thereto including a hollow housing having gasinlet and outlet passages, a port between the passages and a movablevalve member capable of closing and variably opening the port, thehousing having a generally closed gas reception chamber of which thevalve member forms a movable wall, the reception chamber being inrestricted communication with the inlet passage forinlet-gas-pressure-operated closing of said port, said reception chamberhaving a fixed wall transverse to the axis of the port, means forming avacuum chamber in fixed relation to the housing, signal means remotelyof the vacuum chamber and operative to vary subatmospheric pressuretherein, one wall of the vacuum chamber being movable toward and awayfrom said fixed wall in response to the varying subatmospheric pressure,and a pilot valve mechanism for control of said movable valve member bysaid movable wall of the vacuum chamber, said mechanism comprising asubstantially rigid tube in the form of a bell crank resiliently sealedto said fixed wall and passing therethrough for pivotal movement oflever arm constituting portions of the tube toward and away from thevalve member and movable wall of the vacuum chamber respectively, valvemeans carried by the movable wall of the vacuum chamher and co-operatingwith the adjacent open end portion of the tube as a pilot valve to ventgas from said reception chamber at rates in excess of the restrictedrate of reception of gas from the said gas inlet passage, wherebymovements of said valve member are responsive to pilot valving movementsof the movable wall of the vacuum chamber.

References Cited in the file of this patent UNITED STATES PATENTS400,758 Easton Apr. 2, 1889 10 Gassett Dec. 5, 1893 Ljunglof Aug. 20,1907 Campbell Sept. 5, 1933 Stockmeyer Feb. 27, 1934 Schuck et a1 Dec.11, 1934 Adlam Sept. 8, 1936 Betz Nov. 15, 1938 Senninger Apr. 9, 1946Higley May 6, 1947 Swenson et a1 Oct. 23, 1951

